With increasing refinements in and development of medical instruments which are capable of performing substitutive biological functions, increasing attention is being paid to potential hazards and side effects in the use of such instruments. One particularly significant hazard with medical instruments which are directly interconnected to patient tissue, such as the blood stream, is leakage current between the instrument chassis and ground. With an artificial dialysis machine, for example, the patient connected leads create circuits directly into the blood stream and the presence of excessive leakage current is unduly likely to affect the heart of the patient if directed through the patient. While such systems may or may not utilize electrical energy for pumping power, they are required to heat delivered dialysate solution and maintain control of its temperature, and this is conveniently done by electrical means in practical systems. Furthermore, the thermal energy required for heating is substantial, and the heater element is disposed for this reason in the dialysate solution flow path, so the heater element effectively is electrically coupled through the conductive dialysate solution and interchange membrane to the blood stream itself. With conventional 50 or 60 Hz alternating current excitation of the electrical circuits, it can be seen that leakage current can arise from ohmic coupling, which provides a component that is in phase with the exciting signal, and capacitive coupling which provide other components having a fixed phase relation to the exciting signal.
It is wholly inadequate under these circumstances to assume that adequate grounding of the chassis will provide the necessary degree of safety for the system and the patient. The grounding connection may become damaged or broken, either at the machine or otherwise, and the desired maximum of leakage current should not be exceeded under any conditions of possible energization of the circuitry. Thus, regardless of the manner in which the prongs of the connector plug for the power line are inserted into the socket, the leakage current should remain below the desired maximum level. Further, experience has shown with observation of many dialysis machines that the ohmic coupling component is highly variable between different machines, and variable with time and operating conditions even for a specific machine. Workers in the art have heretofore had substantial difficulty in decreasing total leakage current levels to less than 100 microamps.